The present invention is concerned with process analytics, in the case of which a concentration of a chemical substance in water, especially in wastewater or drinking water, is examined. From DE 10 2004 015 387 A1, a method for photometric determination of the total hardness of aqueous solutions is known, in the case of which a reagent is mixed with a sample of an aqueous solution to be examined. The reagent causes, in such case, a color change of the solution to be examined, which is then subjected to a photometric measuring. By means of the measured absorption of the light, the hardness of the aqueous solution is determined.
EP 2 264 449 A1 likewise discloses a photometric method for quantitative determination of an analyte in water, in the case of which, in the process analytical procedure, the sample taking and the metering of the analyte occur, as a rule, fully automatically. In a process analysis device, a chemical substance in water is automatically and virtually continuously photometrically quantitatively determined. In such case, a liquid sample is pumped to a photometric measuring area. On the way thereto, via another pump, a liquid reagent is introduced into the liquid sample stream, which leads to a color change of the liquid sample. The liquid sample is then photometrically examined.
For calibrating or adjusting the analytical devices employed in such a photometric method, standard solutions are used, which contain a predetermined concentration of the chemical substance. Disadvantageous in such case is that, especially in the case of non-linear concentration/signal relationships, a number of standard solutions are required for calibrating or adjusting the analytical device. In such case, depending on how high the concentration of the chemical substance in the liquid to be examined is, one of the standard solutions must be selected, which has a concentration of the chemical substance, which comes nearest to the concentration of the chemical substance in the liquid to be examined. The selected standard solution is then manually applied into the analytical device as a reference liquid. Then, the calibration or adjusting step is performed. The concentration of the chemical substance in the reference liquid measured during the calibration or adjusting step is compared with the concentration of this reference liquid known a priori. The selection and the manual exchanging of the reference liquid lead in running measurement process in process analytics to extended interruptions of the measurement process.